In my opinion, you first have to separate eventers, who are required to wear protective vests on cross country, and your average hunter/jumper riders who are voluntarily wearing protective vests. The requirements and types of falls are sufficiently different to consider separately. The needs of an eventer moving at speed over a cross country course, which is where they have to wear protective vests, are not the same as those of your typical hunter rider.
If you look at discussions here on the eventing forum, it looks like most posters, or at least the most vocal posters, are vehemently against the wearing of air vests on the grounds that they not only aren’t helpful, they make things worse. I’m not going to argue with them. Some of them know way more than I do and maybe they’re right for their sport and their needs. 
However, my needs are entirely different. I’m an old lady who, as you noted, doesn’t bounce anymore. I don’t jump big fences and I don’t jump at speed. The “add” is right in my, and my horse’s, wheelhouse.
I’ve also become increasingly heat intolerant as I have gotten older so my traditional protective vest, about which I used to say “No it’s not too hot in the summer,” has become intolerably hot unless it’s cold out.
And, while I’m not an expert in the engineering of protective gear, I am sufficiently well educated in study design and statistical analysis to read and evaluate the few studies of air vests that are available.
My assessment of the anecdotal data and the limited study data leads me to conclude that for me the air vest is a good option and I can reasonably expect it to provide protection in the kinds of falls I’m likely to have. I haven’t bought one yet, because, yeah, they’re expensive, but it’s on my shopping list.
TLDR: Yes, I guess the jury is still out. But, I have looked at the available information and evaluated it in the context of my personal requirements, and have concluded that an air vest meets my needs.
Also:
Eventers are required to wear protective vests in cross country. Thus far, the only approved vests are the traditional style. So, if you want to wear an air vest, you may, but the air vest alone is not sufficient to meet the vest requirement ,so you also have to wear an approved vest. People who choose to wear an air vest on top of their regular protective vest do so because they believe the air vest adds additional protection.